Currently, marker means are detected via a detection means (e.g., a camera or ultrasound detector). The marker means typically include three markers that are arranged in a fixed and predetermined location relative to each other and are in particular mechanically connected to each other. The markers attached to the marker means can be passive or active markers, wherein passive markers reflect waves and/or radiation emitted in their direction, and active markers are themselves the origin of the radiation and/or waves. The signals emitted from the (active or passive) markers, which can be wave signals or radiation signals, are detected by a detection device (e.g., a camera).
In order to establish an initial position of the marker means relative to the detection means, the marker means may be moved to provide the detection means with different views of the marker means. On this basis, the location of the marker means relative to the detection means can be determined in a known way, in particular in a spatial reference system. Reference is made in this respect to DE 196 39 615 A1 and the corresponding U.S. Pat. No. 6,351,659, each of which are hereby incorporated by reference in their entirety.
As noted above, a marker means typically has three markers, although it may have more or less than three markers. It can then transpire that, from the perspective of the detection means, one of the markers obscures another of the markers such that the one marker can no longer be detected or can only be partially detected by the detection means, e.g., as viewed from the detection means a second marker lies in an optical shadow of a first marker or is partially covered by said optical shadow.
For a marker means that has three markers, only one marker can be obscured by another marker when all three markers are situated in one plane. If one of the markers is obscured by another marker, the accuracy with which the location of the marker means is detected can suffer significantly and detection can become unreliable. In the medical context, such obscuring should be brought to the attention of the medical personnel and/or the current location of the marker means should not be displayed.